The multistage sprocket assembly of a bicycle's derailleur system comprises at least one larger sprocket and at least one smaller sprocket, in which each of the sprocket has a plurality of teeth and is adapted to engage with a drive chain. During a chain shifting process, the chain is shifted from one sprocket, which is referred to as the original sprocket, to adjacent one, which is referred to as the targeted sprocket, with the guidance of the derailleur. For a smooth chain shifting process, the chain must be well engaged with the teeth of the targeted sprocket after the completion of chain shifting motion instead of striding on the tooth tops of the targeted sprocket and then swiftly falling down to the grooves causing the rider to experience an impact from the peddle. Pertinent patents primarily concentrate on the tooth modification of the sprocket solely for the downshifting or solely for the upshifting motions of the chain. The main objective is to allow a smooth chain shifting process from the smaller sprocket to the larger sprocket, or vice versa, by adjusting the relative position between the larger and smaller sprockets, as well as partially modifying the teeth of the sprockets. Basically, in the sprocket modification approach, the chain in the shifting process is deemed as a cutting tool which removed the interference portion between the chain and the sprockets. The modification method and interference amount are influenced by the sprocket specification and the designed shifting path of the chain. On the other hand, the exact shifting position is controlled by the chamfers on the teeth of larger sprocket which enable the chain to fall back to the original sprocket when the chain is not supposed to escape the original sprocket at that position.
The content of the U.S. Pat. No. 4,889,521, is focused on tooth modification for smooth downshifting process of the chain. Its characteristic is in the trimming on two to three teeth of the larger sprocket which allows the chain to be smoothly shifted to the larger sprocket from the smaller sprocket. The chain links between the larger and smaller sprockets are aligned to be a straight line and its length is equal to an integer multiple of chain pitch. Both will decide the adjustment of the relative position between the larger and smaller sprockets.
Another patent that uses the method of tooth modification on the larger sprocket for the downshifting process of the chain is U.S. Pat. No. 5,409,422. In this invention, the position of each of the chain links between the larger and smaller sprockets during the chain shifting process is first adjusted. Following this, the relative phase angle between the larger and smaller sprockets is obtained. Based on the adjusted relative phase angle between the two sprockets, at least two consecutive teeth of the smaller sprocket and at least three consecutive teeth of the larger sprocket are then modified for a smooth downshifting process of the chain.
In the content of the U.S. Pat. No. 5,192,250, the design is aimed at the smooth upshifting motion of the chain. The main feature is in modifying the teeth of the larger sprocket so that each of the modified teeth forms a supporting portion to raise the chain link. The relative position between the larger and smaller sprockets is set so that the straight-line distance between the center of the last chain roller being supported and the center of one tooth space of the smaller sprocket is np or np-.beta. (where p is the chain pitch, n is an integer, and .beta. is smaller than half the chain pitch).
Generally, the chain shifting motion of a multi-speed bicycle can divided into upshifting and downshifting. A bumpy upshifting or downshifting will cause rider's discomfort during the chain shifting process. In order to design a sprocket assembly that possesses both smooth upshifting and downshifting performance, the following problems have to be solved: (1) After adjusting the phase angle between the larger and smaller sprockets according to the downshifting motion of the chain, can an appropriate phase angle be obtained for the upshifting motion of the chain? (2) Will there be a canceling effect on the function if the modification on the sprocket teeth is done for both the upshifting and the downshifting processes of the chain?
Among the current pertinent patents, the tooth modifications done to the sprockets to improve chain shifting performance are solely focused on the downshifting process or solely on the upshifting process of the chain. There has been no design for the sprocket assembly to provide both smooth upshifting and downshifting process of the chain.